1. Field of the Invention
The present invention relates to an LiNbO.sub.3 thin film optical waveguide device and a process for producing the same. The device exhibits the waveguide function for ordinary ray only or for extraordinary ray only or for the light including extraordinary ray. Moreover, it absorbs short wavelength light (e.g. blue light) only a little.
2. Description of the Prior Art
There are available a variety of modern functional optical waveguide devices such as optical deflectors, optical modulators, and optical switches, which are based on LiNbO.sub.3 crystal because of its outstanding electro-optical constant and nonlinear optical constant.
The optical waveguide of LiNbO.sub.3 is usually formed by the metal diffusion process (which involves the step of selectively diffusing a metal such as titanium into the part where the optical waveguide is formed) or by the ion exchange (proton exchange) process.
The foregoing first process has a disadvantage that the resulting optical waveguide is liable to optical damage on account of the introduction of impurities such as titanium. In other words, a titanium-containing optical waveguide changes in refractive index (and hence becomes poor in characteristic properties) when it encounters a beam of intense light. Since the optical damage is more serious as the wavelength is shorter, it cannot be used for blue light in the visible region.
The foregoing second process also has a disadvantage of deteriorating the characteristic properties (e.g., electro-optical constant and nonlinear optical constant) of LiNbO.sub.3. Moreover, its application is limited to an optical waveguide which is effective only for extraordinary ray.
The above-mentioned two processes, both of which are based on diffusion, do not permit one to independently select the width and depth of the waveguide and the distribution of refractive indices in the waveguide. This leads to a problem associated with difficulties in controlling the dimensions and shape and also in creating the stepwise distribution of refractive indices.
A conceivable way to address the foregoing problems is to form the optical waveguide by liquid phase epitaxy. With liquid phase epitaxy, it would be possible to control the depth with certainty and to create the sharp stepwise distribution of refractive indices.
There has been proposed a process for liquid phase epitaxy that employs a substrate of LiTaO.sub.3 and grows LiNbO.sub.3 on it from the molten solution containing Li.sub.2 O-V.sub.2 O.sub.5 as the flux. This liquid phase epitaxy has a disadvantage that the resulting LiNbO.sub.3 waveguide is poor in film quality and absorbs more light in the short wavelength region (blue light).
The present inventors investigated the reason why the LiNbO.sub.3 thin film formed by liquid phase epitaxy absorbs short wavelength light. It was found that the light absorption is due to the entrance of a vanadium atom into the Nb site in the crystal of LiNbO.sub.3 thin film. In this situation, the vanadium ion has the d electron (in the outer shell electron orbital) whose level is split by the crystal field, and transition from one level to another is the cause of light absorption in a specific wavelength region where light absorption should not occur naturally.
Meanwhile, U.S. Pat. No. 3,998,687 discloses a process for the liquid phase epitaxy of LiNbO.sub.3 from a molten solution of Li.sub.2 B.sub.2 O.sub.4 -Li.sub.2 Nb.sub.2 O.sub.6. This process is intended to grow LiNbO.sub.3 on an LiTaO.sub.3 substrate by liquid phase epitaxy. The resulting LiNbO.sub.3 does not selectively guide ordinary light only.